# Source code for MDAnalysis.selections

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    # R. J. Gowers, M. Linke, J. Barnoud, T. J. E. Reddy, M. N. Melo, S. L. Seyler,
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    """
    Selection exporters
    ===================

    Functions to write a :class:`MDAnalysis.core.groups.AtomGroup` selection
    to a file so that it can be used in another programme.

    :mod:`MDAnalysis.selections.vmd`
        VMD_ selections
    :mod:`MDAnalysis.selections.pymol`
        PyMol_ selections
    :mod:`MDAnalysis.selections.gromacs`
        Gromacs_ selections
    :mod:`MDAnalysis.selections.charmm`
        CHARMM_ selections

    The :class:`MDAnalysis.selections.base.SelectionWriterBase` base class and
    helper functions are in :mod:`MDAnalysis.selections.base`, with the
    exception of `:func:get_writer`:

    .. autofunction:: get_writer
    """
    import os.path

    from .. import _SELECTION_WRITERS

    from . import base
    from . import vmd
    from . import pymol
    from . import gromacs
    from . import charmm
    from . import jmol

    def get_writer(filename: str, defaultformat: str) -> base.SelectionWriterBase:
        """Return a SelectionWriter for `filename` or a `defaultformat`.

        Parameters
        ----------
        filename : str
           name of the output file; the extension is used to guess the file format
        defaultformat : str
           if `filename` does not have an extension, use `defaultformat` instead

        Returns
        -------
        SelectionWriterBase : `type`
            the writer *class* for the detected format

        Raises
        ------
        :exc:`NotImplementedError`
            for any format that is not defined
        """
        format = None
        if filename:
            format = os.path.splitext(filename)[1][1:]  # strip initial dot!
        format = format or defaultformat  # use default if no fmt from fn
        format = format.strip().upper()  # canonical for lookup
        try:
            return _SELECTION_WRITERS[format]
        except KeyError:
            errmsg = (
                f"Writing as {format} is not implemented; only "
                f"{ _SELECTION_WRITERS.keys()} will work."
            )
            raise NotImplementedError(errmsg) from None